Pressure sensitive adhesives, which are generally tacky at room temperature, have the capacity of promoting a strong holding force to a surface upon application of pressure, typically no more than finger pressure. Pressure sensitive adhesives are commonly used on products in the form of tapes or sheets. Such tapes are used in applications such as masking for painting operations, packaging, assembly operations, and the like.
In painting operations, pressure sensitive adhesive tapes are used to mask areas of a surface of automobiles, appliances, etc. adjacent to areas that are to be painted. Once painted, the masked article may be subjected to high temperatures, e.g., 120.degree.-180.degree. C., for a period of time, e.g., 30 minutes to 1 hour, to cure the paint. Once the article is painted and cured, the masking tape is removed. Thus, such masking tapes need to withstand high temperatures without deteriorating, staining, or leaving a deposit on the surface.
A pressure sensitive adhesive is typically characterized by its shear strength, peel strength, and tack, as well as its adhesive transfer, i.e., the amount of adhesive residue remaining after removal of the tape. Ideally, a pressure sensitive adhesive adheres tightly to a smooth nonfibrous surface upon application of light pressure, and yet can be removed without delaminating. Furthermore, desirable qualities of a pressure sensitive adhesive tape include solvent resistance and elevated temperature resistance. Ideally, a pressure sensitive adhesive tape should be removable without leaving an adhesive residue. However, this can be very difficult to accomplish especially if the tape has been exposed to high temperatures. Generally, a pressure sensitive adhesive composition includes an elastomeric polymer for adhesive strength and elasticity and a tackifying resin for tackiness and adhesion. A curing agent, i.e., crosslinking agent, for reaction with the elastomer may be used to render the adhesive more resistant to elevated temperatures, aging, and solvent. Other optional ingredients typically include catalyst(s), i.e., accelerator(s), for increasing the rate of reaction between the elastomer and curing agent, filler(s), antioxidant(s), and stabilizer(s).
Conventional pressure sensitive adhesive compositions often contain a heat curable crosslinking agent and at least 1 part by weight of a catalyst per 100 parts by weight of the elastomer, i.e., rubber, and usually about 5-10 parts per 100 parts of the elastomer. See, for example, British Patent Document 975,971 (published Nov. 25, 1964), U.S. Pat. No. 3,231,419 (issued Jan. 25, 1966), and U.S. Pat. No. 3,535,152 (issued Oct. 20, 1974). It is generally believed that adding a catalyst increases the rate of reaction between the elastomer and curing agent, and therefore increases the cohesive strength of the adhesive.
We have found that using a higher aliphatic mercaptan at a level of at least 0.1 percent by weight results in a pressure sensitive adhesive composition that produces a product that exhibits less adhesive transfer after baking than conventional compositions. Furthermore, in one aspect of the invention the composition does not require a curing step during production of a pressure sensitive adhesive product. This eliminates the additional time, energy, and specialized equipment needed to cure the adhesive during production.